Microfluidics has come a long way to transform the way traditional procedures in molecular biology, medical diagnostics, and drug discovery are performed. Lab-on-a-chip and biochip type devices have drawn much interest in both scientific research applications as well as potentially for point-of-care applications because they carryout highly repetitive reaction steps with a small reaction volume, saving both materials and time. While traditional biochip-type devices utilize micro- or nano-sized channels and typically require corresponding micropumps, microvalves, and microchannels coupled to the biochip to manipulate the reaction steps, these additional components greatly increase cost and complexity of the microfluidic device.
Digital microfluidics (DMF) has emerged as a powerful preparative technique for a broad range of biological and chemical applications. DMF enables real-time, precise, and highly flexible control over multiple samples and reagents, including solids, liquids, and even harsh chemicals, without need for pumps, valves, or complex arrays of tubing. In DMF, discrete droplets of nanoliter to microliter volumes are dispensed from onto a planar surface coated with a hydrophobic insulator, where they are manipulated (transported, split, merged, mixed) by applying a series of electrical potentials to an embedded array of electrodes. Complex reaction steps can be carried out using DMF alone, or using hybrid systems in which DMF is integrated with channel-based microfluidics.
Despite significant advances, both evaporation, particularly in air-matrix DMF, and surface fouling remains issues. Surface fouling occur when components from the reaction mixture irreversibly adheres to surfaces of the microfluidic or DMF device after contacting these surfaces. Surface fouling is a particularly acute problem when operating a higher (e.g., greater than 37° C.) temperatures. Various strategies have been proposed to prevent surface fouling, such as using polymers, glass, and metals to fabricate the microfluidic channels or chemical modification of material surfaces. However, these strategies have had limited success, particularly in the context of DMF, despite efforts to test and fabricate surfaces and surface coatings that are resistant to surface fouling. In some instances, a coating intended to prevent surface fouling may cause undesirable interactions and secondary reactions with the reaction mixture and/or reagents used. In general, it would be desirable to have a simple solution to minimizing surface fouling in microfluidic and DMF devices.
Evaporation is also a concern when performing reactions in an air-matrix DMF device. In general, an air-matrix DMF apparatus may refer to any non-liquid interface of the DMF apparatus in which the liquid droplet being manipulated by the DMF apparatus is surrounded by an air (or any other gas) matrix. As used herein, an air-matrix may also and interchangeably be referred to as a “gas-matrix” DMF apparatus; the gas does not have to be air, though it may be. Evaporation may be especially problematic in air-matrix DMF methods and that heat for a prolonged period of time (e.g., greater than 30 seconds). Evaporation limits the utility of air-matrix DMF, because enzymatic reactions are often highly sensitive to changes in reactant concentration. Largely for this reason, others have attempted to use oil-matrix DMF for biochemical applications, despite numerous drawbacks including: the added complexity of incorporating gaskets or fabricated structures to contain the oil; unwanted liquid-liquid extraction of reactants into the surrounding oil; incompatibility with oil-miscible liquids (e.g., organic solvents such as alcohols); and efficient dissipation of heat, which undermines localized heating and often confounds temperature-sensitive reactions. Another strategy for addressing evaporation has been to place the air-matrix DMF device in a closed humidified chamber, but this often results in unwanted condensation on the DMF surface, difficult and/or limited access to the device, and a need for additional laboratory space and infrastructure.
It has also been proposed to address evaporation by transferring reaction droplets from the air-matrix DMF device to microcapillaries, where they can be heated in dedicated off-chip modules without evaporation problems. However, this complicates design and manufacture of the air-matrix DMF device, and introduces the added complications of microcapillary interfaces and coordination with peripheral modules.
Thus, there exists a need for air-matrix DMF apparatuses and methods that may prevent or limit evaporation and/or prevent or limit surface fouling. Described herein are apparatuses and methods that may address this need.